Could/should probably ignore GPDR requests if your business operations are entirely US based, whether or not anyone from the EU uses your site. US national sovereignty doesn't disappear because the EU says jump. We are not bound by the laws of governments other than our own. You can probably ignore them anyway if you aren't a big company. With millions of these troll letters going around (and probably getting ignored…
If you make money from EU users and are US based you need to be GDPR compliant or they will target you through payment processors and ad networks. If you don't make money from EU users and don't want to be GDPR compliant you should probably just shut them off if you ever want to operate in the EU in the future
How will they do that, and on what legal basis?